This invention relates to electrolytic cells and has particular reference to electrolytic cells in which the cathode is caused to move.
The production of metal foil such as copper foil by the electrodeposition of copper onto a rotating cathode drum is well-known in the art. Because the drum is only slowly rotating (in order that a sufficiently thick layer of metal may be deposited on it to form a sufficiently thick foil), it is difficult to avoid depletion of the ions being deposited on the drum in the zone immediately adjacent the surface of the drum. It is also important that the anode/cathode spacing be as small as possible to reduce resistance losses in the electrolytic cell.
The prior art methods have always utilised inert anodes in an operation which is effectively an electrowinning operation. It has not been possible to use a consumable or semi-consumable anode as changes in the shape of normal conventional solid consumable or semi-consumable anodes would result in variations in the anode/cathode gap which would adversely affect both the quality of the foil being deposited and the efficiency of the operation. Prior art foil-making has always supplied the ions for the electrodeposition step in the form of a solution of the ions normally in water. By contrast, however, the present invention makes possible the use of consumable or slowly consumable anode materials and this has the advantage of a lower cell voltage with a consequent economy in power consumption and also because the gap is a regular gap between the anode and the cathode, high quality deposit is formed.
There has been a proposal to use a rapidly moving belt of a nonconductive mesh in the electrodeposition of foil. This mesh includes hard particles which are said to activate the electrodeposit. Details of this proposal will be found in U.S. Pat. No. 3619400. The mesh is, however, rapidly moving and needs to be moved rapidly to entrain ions of the material being deposited into the small gap between the inert anode and the cathode drum.
Another proposal has been made to use a rotating drum (although in this case not for the production of foil) in which a conforming anode which is permanent has on its surface a fibrous mesh which again incorporates hard activating particles. Details of this proposal are to be found in British Pat. Specification No. 1257541 and in particular with reference to FIG. 5 of that specification. An essential feature of both of these prior art suggestions is the incorporation of the hard particles in the mesh. It was considered that the hard particles activated the surface of the electrodeposit through the generation of new dissolution sites through mechanically distorting the crystal lattice of the metal deposited. Sufficient pressure had to be applied to the matrix to achieve the object. The moving belts described in the British Patent Specification (not relating directly to FIG. 5) had a surface speed of 1000 feet per minute (approximately 300 meters per minute) across the surface of the cathode. It is presumed that this speed was necessary to cause sufficient distortion of the electrodeposit to produce the required activation.
In all cases, the prior art constructions used non-consumable anodes which were spaced from the cathode by a very small distance. Other examples of the use of the abrasive belts and pads will be found in U.S. Pat. Nos. 3619401, 3619389, 3619383 and 3619384.